Knee-cap Instability

Having an unstable or dislocating knee-cap can be a very debilitating condition. When a knee-cap first dislocates it is often after a twisting injury when bending the knee. If managed appropriately, the majority do not recur after that first episode. However, a minority go on to develop long-term instability. If physiotherapy is unsuccessful, the next step in treatment is usually surgery.

The patellofemoral joint seen from above at arthroscopy with an easily dislocatable patella
Knee Cap Instability

What is the patella?

The patella (or knee-cap) is a bone that sits at the front of the knee
It has a very important role in the function of the knee:

  1. It improves the efficiency of the quadriceps muscles by lengthening the moment arm i.e. it makes the muscles that extend the knee able to create more force.
  2. It decreases friction at the patellofemoral joint.
  3. It improves stability of the knee.
  4. It centralizes the quadriceps muscle pull.

The stability of the patella is provided by:

  1. The anatomy of the trochlear groove (the indentation/concavity of the femur part of the patellofemoral joint) and patella
  2. Static tension in the soft tissues of the medial and lateral retinaculum – on the inner side of the knee this includes the medial patellofemoral ligament. This is a band of tissue that is very important in preventing the knee-cap sliding to the outer side.
  3. The dynamic control of the quadriceps muscle – the vastus medialis obliqus muscle is especially important in maintaining patello-femoral balance and normal tracking.

The knee-cap sitting to one side of the “trochlea groove”
Knee Cap Instability

Acute injuries to the patello-femoral joint include direct trauma, subluxation or dislocation, patellar fracture, quadriceps tendon or patellar ligament rupture.

Why does patella instability occur?

Knee-cap (or patella) instability is more common when:

  • The groove in which the patella sits is less “dished” – occasionally the groove is barely present at all.
  • The knee is not aligned properly due to a rotational abnormality in the lower limb.
  • The patella has been dislocated or unstable in the past – usually when this happens the medial patellofemoral ligament is damaged (-some operations to stabilise the knee-cap are based on the reconstruction of this structure, see below).
  • There is patella maltracking or not running correctly up and down on bending and straightening. This may lead to excess overload with sheer forces being applied to the articular cartilage in some areas while underloading may occur elsewhere.

How is patella instability treated?

Treatments for patella instability include:

  1. Physiotherapy with or without bracing.
  2. Activity moderation.
  3. Correcting “patella tilt” (if present) with a release of the outer side soft tissues or tightening up the tissues on the inner side.
  4. Realignment procedures.

Two post-operative X-rays showing 2 methods of re-alignment in the same knee

1. The tibial tubercle (bony insertion point of the patella tendon) has been moved to one side to straighten up the pull of the thigh muscles and re-attached with 3 screws

Knee Cap Instability

2. A soft tissue structure called the medial patello-femoral ligament (MPFL) has been reconstructed with a graft taken from the patient’s own hamstrings. The MPFL functions like a check-rain on the knee-cap – preventing it from slipping to the outer side.
Knee Cap Instability